1. Field of the Invention
This invention relates to transport mechanisms for rapidly moving and positioning semiconductor wafers. More specifically, this invention relates to methods and devices for damping vibrations in a handling arm used in such a transport mechanism.
2. Description of Related Art
In modern high speed semiconductor manufacturing operations semiconductor wafers are moved into position for processing with an automated transport mechanism. The transport mechanism uses a handling arm to pick up the wafer and move it into the desired position. One end of the handling arm is connected to the driving portion of the transport mechanism and the other end holds the wafer as it is moved.
One limitation on the speed of processing is the speed with which the handling arm can position and remove the wafer from the processing station. The rapid motion necessary to achieve high speed operation excites vibrations in the handling arm. These vibrations must be allowed to dissipate at least partially before the wafer can be picked up or set down. If the vibrations are not allowed to dissipate sufficiently, positioning accuracy deteriorates. In addition, excess vibration results in particle generation due to impacts between the arm and wafer. This introduces contaminants into the processing chamber.
Existing semiconductor wafer handling arms are not provided with vibration dampers due to the difficulty of finding vibration absorbing materials that are suitable for use in semiconductor processing environments. Semiconductor processing environments are typically characterized by wide pressure and temperature ranges and normally have very low contaminant requirements imposed upon them.
Materials commonly used in other applications to absorb vibrations outgas at low pressures. This creates a source of contaminants that is unacceptable in the ultra-clean environment needed for semiconductor wafer processing. In addition, the damping properties of conventional damping materials vary significantly across the temperature range required to process wafers. This makes them a poor choice for use in such an environment.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a vibration damper for a handling arm that does not use conventional vibration damping materials with their associated contaminant risk and which provides vibration damping for the handling arm over a wide range of processing environment temperatures.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in the art, are achieved in the present invention which is directed to a vibration damper for a wafer handling arm that includes a mass and a spring connected between a response point on the wafer handling arm and the mass to form a mass spring system. The mass spring system has a desired vibration frequency set by the characteristics of the spring and the mass and that vibration frequency corresponds to a bending mode vibration frequency of the wafer handling arm to be damped by the vibration damper.
The spring is preferably a cantilever beam spring, although springs of other types may be used. In the preferred design, the mass is slidably mounted on an end of the cantilever beam spring to permit adjustment of the length of the spring and thereby adjust the vibration frequency for the mass spring system.
Adjustment is provided through a slot in the cantilever beam spring. The mass slides along the slot to permit adjustment of the vibration frequency for the damper. In the most highly preferred embodiment, the mass includes a channel sized to match the cantilever beam spring, the mass being guided along the cantilever beam spring by engagement between the channel and the cantilever beam spring.
The wafer held by the wafer handling arm defines a plane and the cantilever beam spring and mass are preferably oriented to vibrate out of the plane defined by the wafer handling arm to damp out of plane vibrations of the wafer handling arm corresponding to the second out of plane bending mode for the arm.
The spring is constructed of a material that has substantially temperature insensitive spring characteristics over a temperature range suitable for semiconductor processing, such as stainless steel. The mass is also preferably constructed of stainless steel. The materials selected for the damper also should not outgas at low pressures found in a semiconductor processing environment.
The present invention also includes the method of damping vibrations in a wafer handling arm. The preferred method includes the steps of:
selecting a spring constructed of a material suitable for use in a semiconductor processing environment;
selecting a mass constructed of a material suitable for use in a semiconductor processing environment;
positioning the mass on the spring to form a mass spring system having a desired vibration frequency, the vibration frequency of the mass spring system corresponding to a bending mode vibration frequency of the wafer handling arm to be damped by the vibration damper; and
attaching the mass spring system to the wafer handling arm at a high response point for the bending mode of the wafer handling arm to be damped by the vibration damper.